Retina 4K, 21.5-inch, 2019 iMac slow when I upgrade to Big Sur for GarageBand

I have recently wiped my drive and downgraded to Catalina because my computer was starting to get slow. When I turn on the computer and click Safari, It can take almost 5 minutes to open. My computer had around 600 GB used out of its 1 TB. When I downgraded using internet recovery, (great feature by the way) my experience was way smoother and faster. The reason why I upgraded in the first place is because I wanted to install GarageBand for use with a MIDI Keyboard I got recently. But when you go in the App Store, It says the following, "GarageBand can't be installed on "Macintosh HD" because macOS version 11 or later is required." Apple, if you are reading this, may you please add a page on your site to download older versions of GarageBand like 10.3.5.